The use of exhaust gas recirculation (EGR) for reducing NOx emissions from internal combustion gasoline engines has been practiced in the automotive industry for over twenty years. More recently, the diesel engine industry has stepped up its development of EGR systems to meet ever-increasing NOx emissions regulations.
External EGR systems are defined as those systems that extract exhaust gas from the engine's exhaust system and then route it, external to the engine's combustion chamber(s), to the engine's fresh air intake system. To create the necessary flow rate of EGR gases, the EGR must be pressurized. One method for pressurizing the EGR is to extract the EGR gas from a high-pressure portion of the exhaust system and deliver it to a lower pressure portion of the engine's air intake system. The relative pressure difference between the extraction location and the delivery location creates the required mass flow rate.
In the automotive industry, where spark-ignited engines are predominant, the pressure at the air intake is low, because the engine's fresh airflow is restricted by an intake throttle. Hence, the intake system pressure is lower than the exhaust pressure for most operating conditions, and EGR flows readily.
In the diesel industry, most modern engines are turbocharged, meaning that the exhaust and intake systems are pressurized. For best fuel efficiency, it is desirable to have intake system pressure higher than exhaust system pressure, commonly termed “positive engine pressure ratio”. This creates positive pumping work, derived from the turbocharger's use of waste exhaust heat, thus increasing cycle efficiency. Use of EGR on turbocharged diesel engines has been detrimental to fuel efficiency because the positive pressure ratio across the engine must be reversed, so that a negative pressure gradient is formed to create the necessary EGR flow rate. The final outcome is reduced NOx emissions at the expense of fuel efficiency.